The photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving a latent electrostatic image in the non-exposed areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided electroscopic development particles onto the surface of the photoconductor.
The use of vitreous selenium, as described in Bixby U.S. Pat. No. 2,970,906, remains the most widely used photoreceptor in commercial reuseable xerography. Vitreous selenium is capable of holding and retaining an electrostatic charge over relatively long periods of time when not exposed to light, and is relatively sensitive to light as compared to most other photoconductive materials. In practice, vitreous selenium is usually given a positive surface charge during the electrical sensitizing operation. This positive charging takes advantage of the better hole conduction through the selenium layer during illumination, in that selenium has a much more efficient discharge for hole as compared to electrons.
In electrophotography, a photoconductive insulating layer is used which performs dual functions, namely charge generation and charge transport. In the most commonly used process, the functions are performed by a single layer, such for example as a layer of vitreous selenium.
In general, photoreceptor systems used in electrophotographic copying processes are required to exhibit both high dark resistivity for charge retention, and photoconductivity for charge dissipation in response to light activation. Photoconductivity occurs in response to light absorption generating charge carriers in the photoconductor which drift under the influence of the high field placed across the photoconductor in the dark. These charge carriers can be either electrons or holes, depending upon the intrinsic nature of the photoconductive layer. Rarely can a photoreceptor exhibit equal or nearly equal photoconduction through both carrier types. As a result, a photoreceptor layer cannot function equally well when charged negatively or positively.
This is illustrated in FIG. 1 of the drawings which represents the phenomenon occurring in the operation of a normal selenium photoreceptor having a thin layer of vitreous selenium on a conductive substrate. Selenium is a material which offers high mobility to positive charges or holes. In the normal mode, positive charges are established over the surface of the photoconductive selenium layer by corona discharge to charge the plate. As shown in FIG. 1, upon exposure, photons emanating from the tungsten light in the exposed areas will create hole and electron pairs just below the selenium top surface.
The electrons created by the photons will discharge the positive charges on the surface while, on the other hand, holes move rapidly through the selenium layer to the grounded conductive backing. Thus a substantial surface voltage differential is established between the light struck areas and the non-light struck areas of the selenium photoreceptor to provide a latent electrostatic image capable of visual development.
On the other hand, if negative charges are distributed over the surface of the photoconductive selenium layer, as illustrated in FIG. 2, the holes created by the photons emanating in the light struck areas remain to neutralize the negative charges on the surface of the selenium photoconductor. The companion electrons do not transport through the selenium but instead remain trapped just under the surface and present a potential nearly identical to that of the above surface charges that were neutralized. Thus, the voltage differential between the light struck areas and the non-light struck areas is insufficient to define a latent electrostatic image capable of clear visual development.
Attempts have been made to produce a photoreceptor capable of being negatively charged and light decayed upon exposure to radiation. In U.S. Pat. No. 4,026,703, description is made of a dual layered photoreceptor employing vitreous selenium as a photosensitizing layer and a polymeric carbazole derivative as a charge retaining-charge transport layer overlying the selenium photosensitizing layer. In U.S. Pat. No. 3,861,913, description is made of a photoconductor formed of a selenium photosensitizing layer on a conductive substrate and a charge transport layer of tellerium, arsenic and selenium on the selenium photosensitizing layer.
FIG. 3 illustrates the assembly described in U.S. Pat. No. 4,026,703 wherein a top layer of organic photoconductive material, in the form of a polyvinyl carbazole, is provided on a thin layer of vitreous selenium which covers a conductive substrate 24.
In normal use, as illustrated in FIG. 3, the double layer photoreceptor is provided with an overall negative surface charge. The polyvinyl carbazole layer is transparent to white tungsten light such that the light can penetrate through to the underlying photoconductive selenium layer. This selenium layer acts as a charge generator so that the photons of light striking the selenium layer create the described hole and electron pairs. The contact surface between the selenium and the polyvinyl carbazole is designed so that the hole and the electron pairs can be injected across the interface. The holes transport through the polyvinyl carbazole layer to neutralize the negative charges in the corresponding areas on the top surface. The conductive backing is at a positive potential whereby the electrons are drawn thereto for discharge. This provides a substantial voltage differential between the light struck areas and the non-light struck areas to provide a latent electrostatic image capable of good visual development.
On the other hand, as illustrated in FIG. 4, when the described double layer photoreceptor is positively charged, the electron pairs do not travel through the polyvinyl carbazole layer, since the polyvinyl carbazole layer transports holes and not electrons. As a result, the charges are not neutralized and little, if any, differential in voltage is created at the surface as required for establishing a latent electrostatic image capable of good visual development.
It is an object of this invention to produce and to provide a method for producing a bi-modal photoreceptor capable of being positively charged for us in the standard copier mode or negatively charged for use as in the laser writing mode.